The invention relates to an optical head for scanning a surface, which head is provided with a radiation source, an element comprising a substrate on which an input grating is arranged for coupling the radiation from the source into a waveguide of the element, an output grating arranged on the substrate around the input grating for coupling radiation out of the waveguide and for directing said radiation in the form of a scanning beam onto the surface, and a detection system. An optical head of this type may be used in apparatuses for inspecting surfaces in which, for example, contours in a surface are scanned. An example of such an apparatus is a scanning microscope. The head can also be used in a device for scanning recording layers in an optical record carrier. The head can be used for writing and reading the recording layer.
An optical head of the type described in the opening paragraph is known from European Patent Application no. 0,357,780. In this optical head the radiation reflected by the record carrier is sent back into the waveguide via the output grating. The radiation is subsequently passed through an annular output element to an annular detection system where electrical signals are generated for controlling the focusing and the radial position of the scanning beam, as well as a signal which represents the data read in the record carrier. The output grating of the known head has four segments by means of which two extra focus points are made which are offset along the optical axis with respect to the actual focus of the scanning beam. These extra focus points are used to generate a focus error signal. A drawback of this known optical head is that a part of the radiation does not reach the scanning focus and that the pupil of the scanning beam is not uniformly filled with radiation because the radiation from said four segments of the output grating does not contribute to the scanning beam. This results in a reduction of the intensity of the scanning focus, which has a detrimental influence on the performance of the apparatus having such a head. Moreover, the shape of the scanning focus deviates from the ideal shape, i.e. the Airy shape. The higher energy concentrations far away from the optical axis lead to a decreased quality of the data signal and a larger crosstalk between adjacent tracks.
It is an object of the invention to provide an optical head having a scanning focus with an optimum shape and a maximum intensity.
To this end the optical head according to the invention is characterized in that it is provided with a detector grating arranged around the input grating for deflecting the radiation reflected by the record carrier towards the detection system. By making use of a different method for generating focus errors, all radiation coming from the output grating can be used for the scanning focus, which contributes to the shape as well as to the intensity of the scanning focus. Moreover, the radiation need not be coupled into the waveguide again after reflection on the surface to be scanned, so that radiation losses due to the relatively low coupling efficiency of the current output gratings can be prevented.
It is to be noted that Japanese Patent Appliction 62-103857 describes an optical head which also comprises a substrate with two gratings. This optical head does not have a waveguide so that the distance between the radiation source and the substrate must be considerably larger than in the head according to the present invention in order that the beam from the source fills the gratings completely. Moreover, the shape and function of the gratings in the known head differ from those in the optical head according to the invention.
It is possible to mount the output grating and the detector grating on the same surface of the substrate. However, from a technological point of view an embodiment of the invention is preferred in which the output grating is arranged at that side of the substrate which is remote from the radiation source and which is characterized in that the detector grating is arranged at that side of the substrate which faces the radiation source. It is simpler to mount the gratings at different sides of the substrate.
A further embodiment is characterized in that the radiation source is secured to the substrate. This reduces the number of components in the optical head because it does not require a separate holder for the radiation source which should also be aligned on the holder of the substrate during assembly.
In accordance with a further characteristic feature the detection system is secured to the substrate. This has advantages which are analogous to those obtained when securing the radiation source to the substrate. According to the invention the radiation source and the detection system can be mounted at the one side of the substrate, while the output grating and the detector grating can be mounted at the other side.
A preferred embodiment of the optical head is characterized in that the substrate comprises two interconnected parts, in that the detector grating is located between the two substrate parts and in that the output grating is located at the side, remote from the radiation source, of that substrate part which is furthest remote from the radiation source. Since the two gratings are not arranged on one and the same carrier surface, the optical head can be manufactured in a simpler manner.
In accordance with a further aspect of the invention the detector grating may be designed in such a way that it gives the reflected deflected beam a shape which is suitable for generating a focus error signal, i.e. a signal being a measure of the distance between the scanning focus and the surface to be scanned.
A first embodiment of the optical scanning head with an adapted detector grating is characterized in that the detector grating introduces astigmatism into a deflected beam and in that the detection system comprises a four-quadrant detector for generating a focus error signal.
A second embodiment of such a scanning head is characterized in that the detector grating comprises two grating parts for forming two deflected beams which are focused at the same distance from the detector grating, one of the grating parameters, grating period and orientation of the grating lines, being different for the two grating parts, and in that the detection system for each one of said beams comprises two detectors for generating a focus error signal.
A third embodiment of such a scanning head is characterized in that the detector grating comprises two interlaced patterns of grating lines for forming two deflected beams which are focused at mutually different distances from the detector grating, and for each of said beams is provided with a detection system for determining the diameter of the beam at the location of the detection system.
The output grating may be designed in such a way that it does not only couple radiation out of the waveguide but also focuses the scanning beam on the surface to be scanned. In that case the detector grating should be designed in such a way that it does not only deflect the reflected beam to the detection system but also converts this diverging beam into a converging beam. The requirements to be imposed on the grating may become less stringent if, in accordance with a further characteristic feature, a lens is arranged in the optical path of the scanning beam. The two gratings now only have to change the direction of the radiation through a small angle because the beam is focused by the lens.
A special embodiment of the optical head is characterized in that the detector grating, the output grating and the input grating are each divided into n groups of m identical sectors which are associated with each other and are located symmetrically with respect to each other, in which the corresponding groups of sectors of the three gratings form part of a radiation path for a beam having a given wavelength of the radiation source, and in which n and m are integers which are larger than one. Due to the presence of optical paths which have been optimized for different wavelengths, the optical head can operate at different wavelengths. A variation of the wavelength of the radiation source can then be compensated for.
It is to be noted that German Patent Specification no. 2,516,236 describes an optical component which is divided into circular segments, each segment comprising a hologram. The function and the design of the holograms differ from those of the gratings in the sectors according to the invention. The known holograms form different sub-beams from a single, monochromatic coherent beam. In contrast, the gratings according to the invention form different beams from a non-monochromatic beam by selecting a single wavelength from it.